Several types of field effect transistors (FETs) are available for use at microwave and millimeter wave frequencies in the fabrication of monolithic microwave integrated circuits (MMIC). FETs that operate at these frequencies are generally referred to as high-frequency FETs. These high frequency FETs include metal semiconductor field effect transistors (MESFETs) and high electron mobility transistors (HEMTs). Typically, HEMTs are formed from Group III-V materials such as gallium arsenide (GaAs) or indium phosphide (InP), although other materials may be used. In a HEMT there is a doped donor/undoped spacer layer of one material and an undoped channel layer of a different material. A heterojunction is formed between the doped donor/undoped spacer layer and the undoped channel layer. The doped donor layer has a wider bandgap than the undoped channel layer. Due to the conduction band discontinuity at the heterojunction, electrons are injected from the doped donor/undoped spacer layer into the undoped channel layer. Thus, electrons from the relatively large bandgap donor layer are transferred into the relatively narrow bandgap channel layer where they are confined to move only in a plane parallel to the heterojunction. Consequently, there is spatial separation between the donor atoms in the donor layer and the electrons in the channel layer resulting in low impurity scattering and good electron mobility. A layer characterized by movement of electrons confined to a plane parallel to a junction and good electron mobility is referred to as a two-dimensional electron gas.
There are generally three types of HEMTs. One type is referred to simply as a HEMT, whereas the other types are referred to as pseudomorphic HEMTs or PHEMTs and metamorphic HEMTs or MHEMTs. The differences among the HEMT, PHEMT, and MHEMT are that, in the PHEMT, one or more of the layers incorporated into the device has a lattice constant which differs slightly from the lattice constants of other materials of the device, in the MHEMT, one or more of the layers incorporated into the device has a lattice constant which differs significantly from the lattice constants of other materials of the device. As a result of this lattice mismatch, the crystal structure of the material forming the channel layer is strained. Although this lattice mismatch and the attendant strain makes growth of such devices more difficult than the growth of HEMTs, several performance advantages are obtained. For example, the charge density transferred into the channel layer is increased and high electron mobility and high electron saturated velocity are observed. As a result, the devices can develop higher power levels and can operate at higher frequencies with improved noise properties.
An enhancement-mode transistor is a transistor that blocks the flow of current when no gate-source voltage is applied (also called a normally-off transistor). A depletion-mode transistor is a transistor that allows current to flow when no gate-source voltage is applied (also called a normally-on transistor). Typically, the thickness of the active region upon which the gate contact is formed is different for each of these transistors, with this thickness being smaller for the enhancement-type transistor than it is for the depletion-type transistor. The voltage threshold between the two states of these transitions is known as the pinch-off voltage. The pinch-off voltage of a given device is dependent on the thickness of the active region on which the gate contact is formed.
A power HEMT is a depletion mode HEMT characterized by higher drain operating voltage than a conventional depletion mode HEMT. A power HEMT has, as a result of the higher drain operating voltage, a higher output power density. However, a power HEMT or pHEMT, unlike a conventional depletion mode HEMT or PHEMT, typically requires a double gate recess, having two aligned recesses of different widths. Unless otherwise stated, a reference to a depletion mode transistor in this application does not include power mode HEMTs.
In the fabrication of MMICs, it is desirable to employ depletion mode pHEMTs, enhancement mode pHEMTs, and power pHEMTs. It has been understood in the prior art that differing active layers are required for formation of these three types of pHEMTs. In particular, the active layers must have different configurations of etch stops in order to enable formation of different types of pHEMTs. An active layer suitable for formation of both enhancement mode HEMTs and depletion mode HEMTs was disclosed in U.S. Patent Publication No. 2002/0177261 (Song). However, in devices employing power pHEMTs and either or both of depletion mode and enhancement mode pHEMTs, a second active layer is required.
Referring to FIG. 1, a multi-layer structure of the prior art suitable for formation of an enhancement mode pHEMT is shown in cross section. The prior art structure has substrate 10, which is typically of GaAs. Buffer layer 12 is formed on substrate 10. Buffer layer 12 is typically of GaAs, and may include a superlattice of alternating layers of GaAs and AlAs. A thin Si doping layer 14 is provided by silicon pulse doping on buffer layer 12. Spacer layer 16 is provided on doping layer 14. Channel layer 18 on spacer layer 16 may be of InGaAs, or other Type III-V semiconductor material with a relatively narrow bandgap. A second spacer layer 20 is provided on channel layer 18. Spacer layers 16, 20 are of a material such as AlGaAs, which has a relatively wide band gap compared to that of the material of channel layer 16. Second thin Si doping layer 22 is provided on spacer layer 20. Doped semiconductor layer, or Schottky layer, 24, which may be of the same material as spacer layer 18, but with n minus doping, is on second thin Si doping layer 22. A two-dimensional electron gas layer may be obtained in channel layer 16. Contact layer 24, which may be of GaAs, is formed on doped semiconductor layer 22. Contact layer 24 is adapted for formation of drain and source electrodes (not shown). Suitable recesses are formed by etching into contact layer 24, and gates are metallized in the recesses. Depending on the depth of the recess, either depletion mode or enhancement mode HEMTs may be fabricated.